1. Field of the Invention
The present invention relates to a porous ceramic honeycomb filter consisting of a cordierite ceramic. In particular, the invention relates to an improvement of properties of thermal shock resistance and particulate collecting time of the porous ceramic honeycomb filter for collecting particulates, e.g. soots contained in an exhaust gas from a diesel engine for automobiles, and a method of producing the same.
2. Related Art Statement
Recently, ceramic materials having high thermal resistance and thermal shock resistance have been used in various industrial fields with the advance of industrial technology. As an example, a diesel particulate filter having a honeycomb structure consisting of cordierite ceramic is used for collecting particulates contained in exhaust gas from the diesel engine for automobiles.
The diesel particulate filters may be classified broadly into a high collecting efficiency type for big automobiles and a low collecting efficiency type for small automobiles and are used properly corresponding to various purposes.
Japanese patent Application Publication No. 61-54750 is a prior art relating to a filtering performance of a porous honeycomb filter. The publication discloses that porous ceramic honeycomb filters of the high collecting efficiency type and the low collecting efficiency type can be designed by controlling an open porosity and an average diameter of pores. Furthermore, Japanese Patent Application Laid-open No. 58-70814 discloses that the pressure loss can be decreased by providing large pores having a diameter larger than 100 .mu.m.
The porous ceramic honeycomb filter has three properties such as a particulate collecting efficiency, a pressure loss and a particulate collecting time as a filter. The particulate collecting efficiency and the pressure loss are in a relationship of an inverse proportion to each other, that is the higher the collecting efficiency, the greater the pressure loss and conversely the smaller the pressure loss the lower the collecting efficiency. The relationship varies depending on the kinds of engines to be applied.
The most important property of the filter is the particulate collecting time in which the filter can continuously effectively collect the particulates in the gas with a pressure loss not greater than a predetermined value. If the collecting time is short, an interval between regenerating treatments becomes short. Accordingly, there is a problem that the volume of the filter must of necessity be made large since the collecting time is short.